Abstract:

Objective To observe the infection dynamics of Babesia microti in mice with latent infection after re-infection, immunosuppression, or random transmission to healthy mice. Methods Twelve healthy BALB/c mice at the age of six weeks were intraperitoneally (i.p.) injected with 100 μl peripheral blood collected from infected mice (B. microti density, 20%) and then divided into 4 groups (control group, re-infection group, immunosuppression group and random transmission group, n = 3 in each). Tail blood collection was performed for 28 consecutive days to observe morphological changes of B. microti using Giemsa staining, calculate red blood cell (RBC) infection rate, and establish the mouse model of B. microti latent infection. Then the re-infection group once again received an i.p. injection of infected blood at the same dose as above and the immunosuppressive group received i.p. injection of dexamethasone (an immunosuppressant) for five consecutive days(0.5 mg/day). Orbital blood samples were collected in the random transmission group, which were then i.p. injected into 9 healthy BALB/c mice (1 ∶ 3). Tail blood was collected from the 3 groups of mice for consecutive 28 days to calculate RBC infection rate with microscopy and observe morphological changes of B. microti. Results B. microti parasites were first seen in RBC on day 3 in all the 4 groups. The RBC infection rate peaked on day 7(73.2%, 78.0%, 76.2% and 79.0% in control, re-infection, immunosuppression and random transmission groups, respectively), and gradually declined thereafter till the negative infection under a microscope, suggesting a latent infection phase. In the re-infection group, the rate of re-infection was 0 throughout the 28 days, while in the immune suppression group, re-infection was detected from day 2, reached a peak on day 12 (65.2%), and declined thereafter till 0 on day 22 (a latent infection phase). B. microti were firstly seen on day 4 in the transmitted nine mice, reached a peak on day 12 (35.0%-39.0%), and declined thereafter till the latent infection phase. The morphological changes of B. microti were basically similar among various groups. The early stage of infection was characterized by small ring bodies, while the peak stage was mainly composed of large ring bodies and filamentous bodies. Mmultiple parasitisms were also seen. Conclusion Premunition can be seen in mice with latent infection, which can be a source of infection. Immunosuppression of these mice may be followed by occurrences of similar density change dynamics as that observed after the first infection.

Key words: Babesiosis, Babesia microti, Latent infection, Transfusion infection, Immunosuppression